Lab talk

Oct 11, 2013

Understanding the ultimate limit of quantum dot linewidths

Colloidal quantum dots are potentially useful as artificial atoms for applications in emerging quantum technologies. However, previous measurements indicated that these nanocrystals are prone to significant decoherence (as they transition from quantum to classical behaviour). The origins behind this phenomenon remained a mystery, but researchers at the University of Bordeaux in France now provide a possible explanation. Thanks to a novel light absorption-based technique, which reveals that the decoherence is caused by spontaneous charge noise in the environment surrounding the nanocrystals, decoherence-limited linewidths of approximately one gigahertz have been found. The finding should aid in the design of quantum photonic structures containing nanocrystals.

In the quantum regime, particles can act like waves and interfere with each other. However, this quantum interference vanishes as we approach macroscopic length scales as the particles begin to interact with their environment. Physicists usually try to avoid this phenomenon, which is known as decoherence.

Colloidal quantum dots for their part are plagued by spectral instabilities, known as spectral diffusion, which are detrimental to their application in quantum technologies. Spectral diffusion comes about as the excited quantum dot shifts its emission frequency in response to slight changes in its local environment. The phenomenon is unfortunate since colloidal quantum dots could offer some unique advantages in this field thanks to their being compatible with a wide range of photonic structures and the fact that they can be accurately integrated within these structures. Understanding the origin of spectral diffusion is thus important and would ultimately help researchers mitigate these effects.

A team has now studied the fast spectral diffusion process using a resonant photoluminescence excitation technique in which a narrow-band laser is scanned across an absorption line of a single quantum dot and the signal detected The shape of the measured spectral line can show whether the spectral diffusion is caused by the absorption of a photon or not, and the Bordeaux researchers have found that at the highest resolution the spectral diffusion process does not depend on photon absorption.

In this work, the properties of charge noise in disordered media were used to demonstrate that a single colloidal quantum dot is capable of detecting spontaneous changes in the environmental charge distribution via the quantum confined Stark effect. Such fluctuations were found to be compatible with the gigahertz linewidths previously reported. Fast spectral diffusion in quantum dots can thus be attributed to spontaneous environmental charge noise within the disordered local environment, something that ultimately sets a limit on the linewidth that can be obtained with colloidal quantum dots.

More information can be found in the journal Nanotechnology (in press).

About the author

The research was conducted in the Nanophotonics Group at Bordeaux headed by Professor Brahim Lounis at the University of Bordeaux. Dr Mark Fernee is an invited researcher specializing in the photophysical properties of nanocrystals with a particular emphasis on applications in quantum technologies. Chiara Sinito is a PhD student in the group, who together with Dr Yann Louyer participated in the experiments. Professor Philippe Tamarat is an expert in single molecule and single nanoparticle detection.